Spatially precise optical treatment or measurement of targets through intervening birefringent layers

ABSTRACT

A treatment pattern (such as a focused spot, an image, or an interferogram) projected on a treatment target may lose precision if the treatment beam must pass through a birefringent layer before reaching the target. In the general case, the birefringent layer splits the treatment beam into ordinary and extraordinary components, which propagate in different directions and form two patterns, displaced from each other, at the target layer. The degree of birefringence and the orientation of the optic axis, which influence the amount of displacement, often vary between workpieces or between loci on the same workpiece. This invention measures the orientation of the optic axis and uses the data to adjust the treatment beam incidence direction, the treatment beam polarization, or both to superpose the ordinary and extraordinary components into a single treatment pattern at the target, preventing the birefringent layer from causing the pattern to be blurred or doubled.

RELATED APPLICATIONS

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FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

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APPENDICES

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BACKGROUND OF THE INVENTION

This invention relates generally to optical systems that include controlof the polarization of light from an independent light source, andparticularly to optical treatment or measurement of a target, where thefocus or image of the treatment light must be highly resolved, and wherethe treatment or measurement light traverses a birefringent materialbefore reaching the target. The optical treatment may be an imagingprocess, or a process of working the target by means of a laser. Theoptical measurement may be done with coherent or incoherent light.

Although many optical treatments and measurements are performed onexposed targets, some are performed on targets buried under one or moreintervening layers that are substantially transparent to the treatmentbeam. In this document, a “target” is any location where opticaltreatment or measurement is desired, whether it lies on an interfacesurface or within a bulk material. Bottom-surface ablation, as shown inFIG. 1, involves ablating target film 101 by sending treatment beam 102through superstrate 103 (and sometimes other intervening films).Bottom-surface ablation has the advantage that gravity draws ablatedmaterial 104 from ablation cut 105 downward, away from the workpiece, soit does not re-deposit on the workpiece and need to be cleaned off. Inaddition, if the treatment or measurement beam is Gaussian, the depth offocus (DOF) in the intervening layer is n times the DOF in air, where nis the refractive index of the intervening layer. This makes thefocusing or imaging of the beam on the target less sensitive to surfacecontours and thickness variation of the workpiece.

Other optical treatments that may be performed through interveninglayers include, but are not limited to, marking (as described inHerrmann's PCT Application No. WO0061634), annealing and otherstructural and optochemical changes, pinhole remelting, intentionalcolor-center formation, selective curing, and exposure ofwavelength-specific resists, dyes, and other photosensitive material.Measurements may include profilometry, reflectometry, absorption,microscopy, and refractive-index measurements. Reasons for sending thetreatment beam through an intervening layer may be that the target isnot completely solid and needs to be contained; that the target materialneeds to be sealed away from ambient atmosphere; or that the treatmentmust be tamper-proof Measurements are often done through an interveninglayer to determine whether the application of the intervening layeraltered the characteristics of the underlying layer.

Spatial resolution is critical to some of these optical treatments andmeasurements. These treatments and measurements include forming aspatially precise pattern of light at the target; for instance, afocused spot, an image, or an interference pattern. However, if theintervening layer is birefringent, the treatment or measurement beam maybe split into two polarization components propagating at differentspeeds and angles. These separate components form separate patterns atthe target, so that the resolution of the resulting treatment ormeasurement is degraded.

FIG. 2 is a simplified illustration of this effect in the general case.Incident beam 202, having arbitrary polarization, enters layer 203 atincident locus 204 (shown here as a point for simplicity; for differentbeam 202 characteristics, incident locus 204 could be a line or anarea). If layer 203 were isotropic, the entire beam would refract alongordinary path 212 and form a pattern 214 on target 201. However, iflayer 203 is birefringent, its characteristics include an optic axis210, shown here with arbitrary orientation. When beam 202 enters layer203 at incident locus 204, it splits into two orthogonally polarizedcomponent beams: ordinary component 212 polarized in direction 213, andextraordinary component 222 polarized in direction 223. The componentbeams are refracted at different angles because the effective refractiveindex of layer 203 is different for the ordinary and extraordinarypolarizations. Thus the beam forms two spatially separated patterns atthe target: pattern 214 from ordinary component 212 and pattern 224 fromextraordinary component 222.

Depending on the thickness, the ordinary refractive index, and thebirefringent index difference of layer 203 and the incident angle,convergence angle, wavelength, and polarization characteristics ofincident treatment beam 202, the superposed patterns 214 and 224 mayappear on the target as a single blurred pattern or a doubled pattern,similar to an image viewed through Iceland spar crystal. Even at normalincidence, where the impact of isotropic refractive-index variations onoptical treatment and measurement of buried layers is largely mitigated,a birefringent layer can still split the beams along different paths,forming a double pattern at the target, unless the optic axis happens tobe parallel or perpendicular to the layer surface. For example, a beamintended to form a 25-micron spot on a target through 3 mm of glass witha birefringent index difference of 0.005 (which is fairly small) wasobserved to form two overlapping spots with centers separated by 10microns at the target. Where spot resolution is important to treatmentor measurement quality, this is a significant loss of resolution.

A wide range of materials in current industrial use may form abirefringent layer. Many crystalline materials, including liquidcrystals, are inherently birefringent. Microcrystalline thin films mayalso exhibit some localized birefringence. Glasses and polymers,although usually inherently isotropic, can become birefringent fromfabrication stresses (especially if fabricated in large-sheet form) orpost-fabrication treatments. A prime example is tempered glass.

Tempered glass is preferred for use in glass devices that need to bedurable, such as solar panels, outdoor displays, and architectural orvehicle glass. Compared to annealed glass of the same composition,tempered glass is stronger, more thermally resistant, and less hazardousin case of breakage because it breaks into small cuboid fragments ratherthan irregular shards of varying size. However, the residual strain fromthe rapid, non-uniform cooling that tempers the glass makes itbirefringent. Moreover, the birefringent characteristics are notconstant but vary with location on each individual sheet. Transparentpolymers are also used for some of the same applications but“optical-grade” polymers that are specially constrained for lowbirefringence are relatively costly, especially large pieces. Withpolymers, too, the birefringent characteristics vary within a sheet aswell as between individual sheets and from batch to batch.

“Smart” thin-film structures fabricated on glass and transparent polymerare increasingly popular in a wide variety of applications includingsolar panels, displays and active climate-managing windows and lightingfixtures for buildings and vehicles. Many of these products would alsobenefit from the safety and durability of tempered glass or the low costand convenience of non-optical-grade polymer, if precision opticaltreatment and measurement were possible despite the birefringence.Therefore, a need exists for spatially precise measurement and treatmentof a target through an intervening birefringent layer without thepattern-doubling effects that birefringence tends to induce.

BRIEF SUMMARY OF THE INVENTION

An object of this invention is to optically treat or measure buriedtargets with high spatial precision, despite birefringence in theintervening layers. Accordingly, the invention includes orienting theincident direction of the beam, the polarization of the beam, or bothwith respect to the material optic axis to match one of the fourconfigurations where the ordinary and extraordinary components aresuperposed to form a single pattern having substantially the sameresolution that it would have in the absence of birefringence.

Another object of this invention is to adapt to changes in thecharacteristics of the birefringent layer from location to location onthe same workpiece and from workpiece to workpiece. Accordingly, theinvention includes monitoring the orientation of the optic axis of thebirefringent layer at each desired incident locus and formulating theadjustments that will superpose the ordinary and extraordinarycomponents.

Another object of this invention is to make precision optical treatmentand measurement through birefringent layers an automatable industrialprocess. Accordingly, some embodiments of this invention include acontrol loop that continuously monitors the optic-axis orientation ofthe birefringent layer and drives alignment devices to superpose theordinary and extraordinary components based on the monitored data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of bottom-surface ablation, an example ofone of several optical treatments that is performed through anintervening transparent layer.

FIG. 2 is a simplified illustration of how an intervening birefringentlayer splits an incident treatment beam into ordinary and extraordinarycomponents, which propagate at different angles to form separatepatterns at the target.

FIGS. 3 a-3 d are conceptual diagrams of four configurations of thetreatment or measurement beam with respect to the workpiece, each ofwhich results in the ordinary and extraordinary beams being superposedto form a single pattern at the target.

FIG. 4 illustrates the effect of treatment threshold or measurementnoise level on the results of non-ideal configurations.

FIG. 5 is a conceptual diagram of monitoring embodiment that monitorsthe orientation of the optic axis indirectly by monitoring its effect ona focused spot.

FIG. 6 a is a conceptual diagram of an alternate monitoring embodimentusable when the surfaces of the target and birefringent layer (and anyother intervening layers) are substantially specular and some measurablelight from the monitoring beam is reflected from the target surface.

FIG. 6 b is a conceptual diagram of an alternate monitoring embodimentthat can be used when the target and birefringent layer (and any otherintervening layers) transmit at least some measurable light at awavelength where an image receiver, such as a CCD, is sensitive.

FIG. 7 is an example schematic of a tool adapted to make the treatmentor measurement beam parallel to the optic axis.

FIG. 8 is an example schematic of a tool adapted to make the treatmentor measurement beam perpendicular to both the workpiece surface and theoptic axis in the non-trivial case where the target is not flat and thebirefringent layer is not plane-parallel.

FIG. 9 is an example schematic of a tool adapted to adjust the treatmentor measurement beam polarization to be parallel or perpendicular to theplane shared by the incident beam and material optic axis.

DETAILED DESCRIPTION OF THE INVENTION

This invention leverages four relative configurations of the optic axisof a birefringent material, the incident direction of a light beam, andthe polarization of an incident light beam, which cause all the light tobe refracted at the same angle while traveling through the birefringentmaterial. If all the light is refracted at the same angle instead ofbeing split into two components refracted at different angles, the beamcan form a single pattern at the target, as if the intervening layerwere not birefringent. The four configurations are:

-   -   1. The beam propagates through the birefringent layer in a        direction parallel to the optic axis. Under this condition, an        incident beam of arbitrary polarization will not split into        components traveling in different directions.    -   2. The optic axis is parallel to the entrance surface of the        birefringent material at the incident locus, and the beam enters        at normal incidence. Under this condition, a beam polarized        linearly, parallel or perpendicular to the optic axis, will not        split into components at all. A beam of arbitrary polarization        will split into two beams traveling at different speeds, but        they will travel in the same direction and remain fully        overlapped.    -   3. The beam polarization is linear, oriented parallel to a plane        (the “I-OA plane”) that contains both the incident beam and the        optic axis.    -   4. The beam polarization is linear, oriented perpendicular to        the I-OA plane.

FIGS. 3 a-3 d are idealized illustrations of configurations 1 through 4,respectively. In FIG. 3 a, incident beam 302 a may have any polarizationentering birefringent layer 303 a at incident locus 303 c; if incidentbeam 302 a is parallel to optic axis 310 a, the refracted beam 312 awill form a single pattern on target 301 a. In FIG. 3 b, incident beam302 b is normal to the entrance surface of birefringent layer 303 b, andoptic axis 310 b is in some orientation parallel to that surface atincident locus 304 c. Regardless of its polarization, refracted beam 312b will form a single pattern on target 301 b. In FIG. 3 c, incident beam302 c is linearly polarized in direction 313 c, parallel to the I-OAplane 320 c that contains both optic axis 310 c (which may have anyorientation in that plane) and incident beam 302 c. Refracted beam 312 calso forms a single pattern on target 301 c. In FIG. 3 d, incident beam302 d is linearly polarized in direction 313 d, perpendicular to theI-OA plane 320 c that contains both optic axis 310 d (which may have anyorientation in that plane) and incident beam 302 d. Refracted beam 312 dalso forms a single pattern on target 301 d. The optimal choice amongthese configurations depends on the nature of the birefringent layer(for instance, its shape, its thickness, and the range of its expectedoptic-axis orientations) and on the nature of the beam (for instance,its result tolerances, numerical aperture, and working distance).

One skilled in the art would probably not expect an actualpattern-forming beam to behave as well as the idealized rays in FIGS. 3a-3 d. For example, if a beam is focused on the target through thebirefringent layer to form a converging cone of light, the light entersthe birefringent layer at a range of incident angles rather than asingle angle. The optic axis, by contrast, tends to have a singleorientation, at least over a small area of the workpiece. Therefore, theentire range of beam propagation angles cannot be simultaneouslyadjusted to an ideal angle with the optic axis. Image-forming beams,like focused beams, also propagate at a range of angles rather than asingle angle. Beams forming interference patterns, however, are oftencollimated so that all the parts of the beam propagate at substantiallythe same angle. Another obstacle foreseeable by those skilled in the artis the difficulty, and hence the equipment cost, of adjusting thepolarization and refracted beam to precisely the correct angle.

Contrary to those logical expectations, however, these configurationshave been shown to form very well-resolved patterns with fairly tightlyfocused beams where the polarization and refraction angles are onlysubstantially near the ideal. This is because of inherent limitations inboth treatment and measurement with beams of very low intensity.

FIG. 4 shows the result of slightly non-ideal configuration. An idealconfiguration would produce only main peak 414; a non-idealconfiguration produces a second peak 424 from the second polarizationcomponent, some blurring 415 around main peak 414 from some parts of thebeam coming in at non-ideal angles or both. The dotted line 400represents the low-intensity limit of the process. If the process is anoptical treatment, limit 400 may be a threshold, such as an ablation orreaction threshold. If the process is an optical measurement, level 400could be a noise level of a measurement sensor. Any light reaching thetarget with intensity below limit 400 will not affect the process.Therefore, even though not all parts of the beam are incident at exactlythe ideal angle and the second polarization component 424 is notperfectly extinguished, the treatment or measurement of FIG. 4 willbehave as if only the above-the-limit light—that is, main peak 414—werepresent.

Therefore, if the extinction is “good enough,” and if the incident anglerange is small (e.g., if the beam is focused or the image is formed at alow numerical aperture) and the birefringent index difference is smallor the birefringent layer is fairly thin, experiments show that apattern of acceptable resolution is formed at the target if thepolarization is reasonably linear and properly oriented and thepropagation direction is corrected in the center of the beam. Beams thatcarry most of their intensity in the center, such as the Gaussian beamsproduced by many lasers, are generally more forgiving of “center-only”propagation-direction correction than beams that have more intensityaround the edges.

The low-intensity limit will depend on the wavelength of the beam, itstime-dependent intensity characteristics (e.g. pulse profile), and thenature of the target material and any measurement sensor. However, theselimits may be available in product specifications or technicalpublications, or they be measured with reasonable ease; therefore, theycan be derived without undue experimentation.

Monitoring the orientation of the optic axis can be important forachieving acceptable results, especially if the optic axis orientationvaries from workpiece to workpiece or from one incident locus to anotheron a single workpiece. A number of approaches to this measurement exist.Because the monitoring method can be independent of the chosenconfiguration, they will be discussed separately here.

In a preferred embodiment shown in FIG. 5, the orientation of the opticaxis is measured indirectly by measuring its effect on a focused spot.Monitor beam 532 should be a wavelength that behaves analogously to thetreatment or measurement beam; it may even be a sub-threshold attenuatedfraction of the treatment beam or the actual measurement beam. Monitorbeam 532 enters through birefringent layer 503 and focuses, if theconfiguration is not ideally aligned, to two spots 514 and 524 on target501. Camera 551 forms an image 552 of the two spots on an imagereceiver, which may be a CCD array or appropriate equivalent. This imageis an intensity map, from which such quantities as the relativeintensities I₁ and I₂ of the two spots, their separation d₁₂, and thewidth w₁ of the brightest spot at a predetermined cut-off point can bedetermined. This monitored data can be compared with stored data, suchas the maximum allowable ratio I₂/I₁, or the maximum allowable width w1(where, for instance, d₁₂ is so small that only one blurry spotappears). The relative tilt between the beam and workpiece, the beampolarization or both can be manipulated until the measured values fallbelow the stored maxima.

FIG. 6 a is a schematic of an alternate monitoring embodiment that canbe used when the surfaces of target 601 and birefringent layer 603 (andany other intervening layers) are substantially specular and somemeasurable light from the monitoring beam is reflected from the targetsurface. As in FIG. 5, monitoring beam 632 focuses, in the general case,to two spots 614 and 624. However, in this embodiment, imaging lens 653captures the reflected beams and re-focuses them onto image receiver651, which may be a CCD array or appropriate equivalent. Received image652 is analyzed in the same manner as image 552 in FIG. 5, and theworkpiece tilt, polarization or both can be manipulated until the imageparameters are within a pre-determined acceptable range, correspondingto the desired resolution of the treatment or measurement.

FIG. 6 b is a schematic of an alternate monitoring embodiment where themonitoring is done at a wavelength where all the layers of the workpiecetransmit at least a measurable amount of light. Extended light source633 emits light polarized in direction 634. Source 633 may be anincoherent light source masked by a linear polarizer or it may be alight source that “naturally” emits polarized light, such as anactive-matrix LCD display. The light from source 633 is transmittedthrough target layer 601 b and birefringent layer 603, then throughlinear polarizer 663. Linear polarizer 663 is oriented in direction 664to “cross” the polarization of source 633. Imaging lens 654 images lightsource 633 onto image receiver 652. If there were no birefringentmaterial between polarized source 611 and crossed polarizer 663, littleor no light would be transmitted through crossed polarizer 663, andimage receiver 652 would be entirely dark. Birefringence in layer 603,though, changes the polarization of the light so that more light passesthrough crossed polarizer 663 and reaches image receiver 652. The figureillustrates a birefringent layer with spatially varying birefringence;the light pattern on image receiver 652 is non-uniform. If thebirefringence were constant across birefringent layer 603, theillumination would be uniform. Images from the receiver for various canbe fed to the measurement or treatment tool as a map of thebirefringence characteristics of the workpiece.

Numerous methods also exist in the prior art to monitor the optic axisorientation directly, such as polarization-sensitive optical coherencetomography. In embodiments where the optic axis orientation is monitoreddirectly, the analysis would include the necessary adjustments torelative tilt between the beam and the workpiece, incident polarizationangle or both that would produce one of the four desirableconfigurations.

In some embodiments, the monitoring is done either periodically orcontinuously during treatment or measurement, with the monitored dataproviding feedback for a closed control loop that adjusts theconfiguration whenever the monitored data changes to indicate anout-of-tolerance misalignment. Optionally, some embodiments may includeshutters or other devices to extinguish or attenuate the treatment beam,or stop the measurement, if the monitored data approaches the tolerancelimit, then cease to extinguish or attenuate the treatment beam orresume measurement when the configuration is readjusted to a comfortablemargin. In other embodiments, the monitoring system may “map” the areasof workpiece to be treated or measured and store the data before thetreatment or measurement begins. Still other embodiments, for workpieceswhere the orientation of the optic axis is expected to be uniform overthe treatment or measurement, would only require collecting monitoreddata at a single representative incident locus before treatment ormeasurement.

Once the alignment parameters to produce a desired configuration areknown, the measurement or treatment apparatus can produce the desiredconfiguration in a number of ways.

FIG. 7 is an example schematic of a tool adapted to make the treatmentor measurement beam parallel to the optic axis (Configuration 1).Configuration 1 is most advantageous when the optic axis orientation isfairly close to perpendicular to the target surface. Light source 760produces beam 702. Beam-shaping assembly 761 forms the beam into thedesired pattern aimed at target 701. Beam steering assembly 764, whichmay include optics to fine-adjust the translation and angle of beam 702,may also be present in the beam train. Birefringent layer 703 has opticaxis orientation 710 at the locus of incidence. The workpiece (herecomprising target layer 701 and birefringent layer 702) is supported byworkpiece stage 762. Many treatment and measurement tools includeactuators to change the relative position of stage 762 and beam 702 inX, Y, and Z directions 763, either by moving the stage or by moving thelight source and its optics. Because light refracted parallel to opticaxis 710 will form a single spot 714 regardless of its polarization,only the relative tilt of beam 702 with respect to the workpiece needsto be adjusted. Adjusting this relative tilt by changing the angle ofbeam 702 using beam steering assembly 764 can be highly precise but therange can be limited by the aperture or aberration sensitivity ofbeam-shaping assembly 761. By contrast, adjusting this angle by tiltingthe stage in directions 765 can offer a larger adjustment range butbecause the stage is generally much more massive than the beam-steeringoptics; the required actuators will be more expensive and may not be asprecise. The choice of either or both of these ways to adjust the tiltwill depend on the particulars of the workpiece and process types.

Configuration 2, adjusting the refracted beam to be perpendicular toboth the entrance surface of the birefringent layer and its optic axis,is useful in the special case where the optic axis of the birefringentlayer is substantially parallel to the entrance surface of thebirefringent layer. Where the target is flat and all the layers above itare plane-parallel, the birefringence may not even manifest as a problemin common normal-incidence treatments and measurements. However, whenthe target layer is not flat or the birefringent layer is notplane-parallel, the birefringence may cause loss of resolution if noadjustment is done.

FIG. 8 is an example schematic of a tool adapted to use Configuration 2with a workpiece where target 803 is not flat and birefringent layer 803is not plane-parallel. . Light source 860 produces beam 802.Beam-shaping assembly 861 forms the beam into the desired pattern aimedat target 801. Beam steering assembly 864, which may include optics tofine-adjust the translation and angle of beam 802, may also be presentin the beam train. Birefringent layer 803 has optic axis orientation 810at the locus of incidence. Stage 862 supports the workpiece. Manytreatment and measurement tools include actuators to change the relativeposition of stage 862 and beam 802 in X, Y and Z directions 863, eitherby moving the stage or by moving the light source and its optics.Because light refracted parallel to optic axis 810 will form a singlespot 814 regardless of its polarization, only the relative tilt of beam802 with respect to the workpiece needs to be adjusted. Here, it needsto be adjusted whenever the angle of the entrance surface ofbirefringent layer 803 changes enough to unacceptably degrade theresolution of the treatment or measurement. Adjusting this relative tiltby changing the angle of beam 802 using beam steering assembly 864 canbe highly precise, but the range can be limited by the aperture oraberration sensitivity of beam-shaping assembly 861. By contrast,adjusting this angle by tilting the stage in directions 865 can offer alarger adjustment range, but because the stage is generally much moremassive than the beam-steering optics, the required actuators will bemore expensive and may not be as precise. The choice of either or bothof these ways to adjust the tilt will depend on the particulars of theworkpiece and process types.

Configurations 3 and 4—adjusting the polarization to be parallel orperpendicular, respectively, to the I-OA plane formed by the centralaxis of the refracted beam and the optic axis of the birefringent layer,are the most useful when the optic axis is neither substantiallyparallel nor substantially perpendicular to the entrance or targetsurfaces. However, they impose an extra requirement: that thepolarization characteristic of the beam be substantially linear, andthat some adjustment can be made to orient the polarization eitherparallel or perpendicular to the I-OA plane without unacceptabledistortion of the pattern from other sources, such as aberrations in thebeam-shaping optics.

FIG. 9 is an example schematic of a preferred embodiment of a tooladapted to adjust the treatment or measurement beam polarization toConfiguration 3 or 4. Light source 960 produces beam 902. Some lightsources, such as some types of laser, produce a beam that is alreadysubstantially linearly polarized and collimated. Beam-shaping assembly961 forms the beam into the desired pattern aimed at target 901. Beamsteering assembly 964, which may include optics to fine-adjust thetranslation and angle of beam 902, may also be present in the beamtrain. Stage 962 supports the workpiece. Many treatment and measurementtools include actuators to change the relative position of stage 962 andbeam 902 in X, Y and Z directions 963, either by moving the stage or bymoving the light source and its optics. Birefringent layer 903 has opticaxis orientation 910 at the locus of incidence. When beam 902 entersbirefringent layer 903, it is refracted. The center of the refractedbeam and the local optic axis lie in a plane, the I-OA plane 920. If thepolarization of beam 902 is linear and either parallel (Configuration 3)or perpendicular (Configuration 4) to I-OA plane 920, a single effectivepattern 914 with optimized resolution will be formed at target 901.Polarization adjuster 967 manipulates the polarization of beam 902 toachieve this.

Most polarization adjustment devices work best with collimated light.Some light sources, such as many lasers, produce light that is alreadysubstantially collimated and linearly polarized. For this type of lightsource, polarization adjuster 967 may be a rotatable half-wave plate, orany other device for rotating the orientation of a linear polarization.If the light source is collimated, but circularly or randomly polarized,polarization adjuster 967 may be a rotatable linear polarizer (althoughthis method can significantly attenuate the beam). If the light sourceis not collimated, a collimator 966 may be inserted in the beam trainbefore polarization adjuster 967.

The FIG. 9 embodiment is preferred because it does not requiredisturbing the nominal angle at which the beam reaches the target' toomuch change in this angle can cause the treatment or measurement patternto be distorted in some cases. However, if distortion is not asignificant risk, alternate embodiments can use a linearly polarizedbeam of constant orientation and adjust the relative angle between thebeam and the entrance surface of the birefringent layer, as in the toolsillustrated in FIGS. 6 and 7, until the polarization is either parallelor perpendicular to the I-OA plane.

To summarize, this invention mitigates or eliminates the loss ofresolution that can occur when a treatment or measurement beam passesthrough a birefringent layer before reaching its target. Adjustments tothe relative angle of the beam axis and workpiece, or to the beampolarization, or both, produce one of four configurations that minimizethe resolution loss from the birefringent layer: (1) beam axis parallelto the optic axis of the birefringent material, (2) beam axisperpendicular to both optic axis and entrance surface of thebirefringent layer, (3) polarization parallel to the I-OA plane sharedby the beam axis and optic axis, or (4) polarization perpendicular tothe I-OA plane shared by the beam axis and optic axis. The adjustmentsare chosen in response to monitored data about the orientation of theoptic axis or about the effects of that orientation on a monitoringbeam. Monitored data to determine the optimum adjustment can becollected before or during treatment or measurement. Monitored data canbe the input, and adjustment commands the output, of a closed controlloop.

Those skilled in the art will recognize that only the claims, not thisdescription or the accompanying drawings, limit the scope of theinvention.

1. An apparatus for optically treating or measuring a target through abirefringent layer having an optic axis, comprising: a light sourceadapted to generate at least one beam having an incidence direction anda polarization characteristic; a monitoring apparatus for monitoring anorientation of the optic axis or effects that indicate the orientation,and an alignment apparatus for adjusting the relative orientation of theoptic axis and at least one of the incidence direction and thepolarization characteristic based on the monitored data; where the beamforms at least one pattern on the target; the beam enters thebirefringent layer at an incident locus; the birefringent layer cansplit beams with some combinations of incidence direction andpolarization characteristic into ordinary and extraordinary componentspropagating at different angles, and an optimal adjustment of thealignment apparatus creates an acceptably resolved pattern at thetarget.
 2. The apparatus of claim 1, where the optimal adjustment causesthe beam to be refracted substantially parallel to the optic axis of thebirefringent layer at the incident locus.
 3. The apparatus of claim 1,where the optimal adjustment causes the beam to be refractedsubstantially perpendicular to the optic axis of the birefringent layerat the incident locus.
 4. The apparatus of claim 1, where thepolarization characteristic is substantially linear at a polarizationangle, and the optimal adjustment causes the beam to be refractedsubstantially in the same plane as the dominant polarization directionand the optic axis at the incident locus.
 5. The apparatus of claim 1,where the polarization characteristic is substantially linear at apolarization angle, and the optimal adjustment causes the polarizationangle to be substantially parallel to the plane of the incident beam andthe optic axis at the incident locus.
 6. The apparatus of claim 1, wherethe polarization characteristic is substantially linear at apolarization angle, and the adjustment causes the polarization angle tobe substantially perpendicular to the plane of the incident beam and theoptic axis at the incident locus.
 7. The apparatus of claim 1, where themonitoring apparatus collects monitored data from the workpiece beforethe treatment or measurement begins, a storage medium stores themonitored data, and the alignment apparatus retrieves the monitored datafrom the storage medium and uses the monitored data to calculate andperform adjustments during the treatment or measurement.
 8. Theapparatus of claim 7, where the monitoring apparatus collects monitoreddata for the entire area of the workpiece to be measured or treated. 9.The apparatus of claim 1, where the monitoring apparatus collectsmonitored data from the workpiece during the treatment or measurement.10. The apparatus of claim 9, further comprising a controller adapted toperform functions comprising: collecting data from the monitoringapparatus, calculating the optimal adjustment of the alignment apparatusto form the beam into a focused spot, image, or interference pattern ofpredetermined characteristics on the target, and commanding thealignment apparatus to make the optimal adjustment.
 11. The apparatus ofclaim 10, where the controller is adapted to perform further functionscomprising: attenuating or extinguishing the beam in response tomonitored data that indicates that the adjustment is nearly out oftolerance, and ceasing to attenuate or extinguish the beam when anoptimal adjustment of the alignment apparatus is complete.
 12. Theapparatus of claim 1, where the monitoring apparatus monitors theorientation of the optic axis indirectly by monitoring the polarizationof light reflected, diffracted, or scattered through the birefringentlayer.
 13. The apparatus of claim 1, where the monitoring apparatusmonitors the orientation of the optic axis indirectly by monitoring thecharacteristics of a beam of light reflected, diffracted, or scatteredthrough the birefringent layer.
 14. The apparatus of claim 1, where themonitoring apparatus monitors the orientation of the optic axis bymapping the image of an extended polarized light source as viewedthrough the target, birefringent layer, any intervening layers, and apolarizer oriented to extinguish the light emitted directly from theextended polarized light source.
 15. The apparatus of claim 14, wherethe extended polarized light source is an LCD screen.
 16. The apparatusof claim 1, where the monitoring apparatus monitors the orientation ofthe optic axis indirectly by analyzing the image of an optical patternformed on the target through the birefringent layer.
 17. A method ofoptically treating or measuring a target with a beam having an incidencedirection and a polarization characteristic, through a birefringentlayer having an optic axis, where the birefringent layer is capable ofsplitting incoming light in some incidence directions and polarizationcharacteristics into ordinary and extraordinary components, comprising:monitoring the orientation of the optic axis at an intended incidentlocus of the beam on the birefringent layer, and adjusting the anglebetween the optic axis and at least one of the incidence direction andthe polarization characteristic, so that a beam entering thebirefringent layer at the incident locus will not be split into ordinaryand extraordinary components that both exceed a treatment threshold ormeasurement noise level relevant to the treatment or measurement beingperformed.
 18. The method of claim 17, where “adjusting” causes the beamto be refracted substantially parallel to the optic axis of thebirefringent layer at the incident locus.
 19. The method of claim 17,where the optic axis of the birefringent layer is substantially parallelto the entrance surface of the birefringent layer at the incident locus,and “adjusting” causes the beam to be refracted substantiallyperpendicular to the optic axis of the birefringent layer at theincident locus.
 20. The method of claim 17, where the polarizationcharacteristic is substantially linear in a dominant polarizationdirection, and “adjusting” causes the beam to be refracted substantiallyinto the plane of the dominant polarization direction and the opticaxis.
 21. The method of claim 17, where the polarization characteristicis substantially linear in a dominant polarization direction, and“adjusting” causes the dominant polarization direction to besubstantially parallel to the plane of the incident beam and the opticaxis at the incident locus.
 22. The method of claim 17, where thepolarization characteristic is substantially linear in a dominantpolarization direction, and “adjusting” causes the dominant polarizationdirection to be substantially perpendicular to the plane of the incidentbeam and the optic axis at the incident locus.
 23. The method of claim17, where “monitoring” and “adjusting” are functions of an automaticcontrol loop having stored data on tolerable separations between theordinary and extraordinary components at the target.
 24. The method ofclaim 17, where “monitoring” produces data about the optic axisorientation indirectly from the characteristics of light reflected,diffracted, or scattered through the birefringent layer.
 25. An articleof manufacture made by optically treating a target, with a beam havingan incidence direction and a polarization characteristic, through abirefringent layer having an optic axis that splits treatment light withsome incidence directions and polarization characteristics into ordinaryand extraordinary components, where: the ordinary and extraordinarycomponents are superposed at the target by monitoring the orientation ofthe optic axis, and adjusting at least one of the incidence directionand the polarization characteristic to a desired angle relative to theoptic axis.
 26. An article of manufacture, comprising acomputer-readable storage medium programmed with instructionscomprising: calculation of optic-axis orientation of a birefringentlayer from incoming monitored data, calculation of the adjustment of therelative angle between the optic-axis orientation and at least one of anincidence direction and a polarization characteristic of a beam thatwill superpose the ordinary and extraordinary components into which thebeam would otherwise be split, and generation of commands to at leastone alignment apparatus to make the calculated adjustments.
 27. Thearticle of manufacture of claim 26, further comprising instructions for:attenuating or extinguishing the beam, and returning the delivered beamto full intensity, power, or energy.